Apparently not occurring as a wild species, the bengalese is thought to be an hybrid, bred by the Chinese. If this is so they did a marvellous job. These little birds are second to none when it comes to parental duties. Not only do they work hard to rear their own young, but they also make wonderful foster parents for rearing other similar sized finches.
Sex can really only be determined by watching for the cock birds singing, accompanied by assuming the shape of an inverted triangle and bouncing up and down along the perch. The best way to force a display is to cage the bird in question alone for a couple of days, then introduce another bird into the same cage. If the first bird is a cock it will almost immediately start singing, usually regardless of the sex of the second bird.
Colours are somewhat limited at the moment being various shades of brown, but this ranges from pale cream to dark chocolate. Add to this the fact that all shades can be plain or pied and crested or plain headed, plus they also occur as albinos.